A measuring apparatus of the state of the art for measuring the voltage of a measurement signal of a measuring element, e.g. a sensor element such as, for instance, a glass sensor for measuring the pH-value, is illustrated schematically and simplified in FIG. 1. From the measuring element (not shown), the measurement signal, having an input voltage UE, is conducted through a connection region of a measuring apparatus via the measurement voltage input 1, having at least one input contact 2 and a reference-input contact 7. The connection region in such case must have at least the input contact 2. The reference-input contact 7 is a reference potential, or is connected with a reference potential (e.g. ground). The input contact 2 is connected with a high-resistance input of a voltage follower/impedance converter 8. Following the impedance converter 8, the measurement signal having a measurement voltage UM can be further processed at low resistance, for example by additional analog signal processing, analog to digital conversion, etc. This will not be described here in greater detail, and is known to those skilled in the art. By means of the high resistance of the impedance converter 8 with its high-ohm input resistance RI, voltage signals of a measuring element with large internal resistance can also be measured, such as, for example, those of glass sensors used for measuring pH-value.
There is, however, in the case of such measuring devices of the state of the art, the disadvantage, that the high-ohm, measurement voltage input 1 is sensitive to electrical disturbances. Thus, for example, leakage resistance paths contacting the high-resistance, measurement voltage input 1 are critical. If an ohmic leakage resistance, schematically represented in FIG. 1 by RF, lies for example, in the range of the size of the internal resistance of the measuring element being used, or if it comes close to the range of the size of this internal resistance of the measuring element, a corruption of the signal of the measuring element occurs. As shown schematically in FIG. 1, the leakage resistance RF can be considered as a parallel resistance to the input resistance of the measuring apparatus.
A reason for the formation of the leakage resistance RF at the high-resistance measurement voltage input 1 can be, for example, moisture deposits. In the case of application of an electronic measuring apparatus in the vicinity of liquid media, if a malfunction occurs, a possible cause can be moisture accumulation. This can occur, for instance, in applications in which such an electronic measuring apparatus is applied in liquid analysis as sensor with on-site electronics, i.e. such a measuring apparatus is connected directly and locally with a measuring element, which itself contains liquid media, and/or is operated in liquid media.
If, for example, in the case of such a measuring apparatus, the seal against a liquid medium is defective, for example, through defective mechanical construction or mechanical stress, moisture can get into the electronic measuring apparatus, especially at its measurement voltage input 1. This leads then to a corruption of the measurement results. Thus, in practice, such moisture deposits first form leakage paths with the leakage resistance RF in the region of the input contact 2 and the reference-input contact 7 of the measurement voltage input 1. Since, as a rule, the connection of the reference-input contact 7 to the reference potential (e.g. ground) is of low resistance, the high-resistance input contact 2 is likewise drawn via the leakage resistance RF to this reference potential. If the reference potential concerns a possible measurement potential, then based on the measured voltage in such a case, it can no longer be distinguished whether a moisture deposit at the measurement voltage input 1 of the measuring apparatus is present, and thus consequently a corrupting disturbance, or whether the measuring apparatus can be properly operated, wherein from the measuring element, e.g. an input voltage UE of size in the range of the reference potential is given at the measurement voltage input 1. As a result, a possible leakage resistance RF thus directly influences the measured variable of a measuring element. Consequently, it is desirable in practice to find an opportunity for safely and reliably determining whether a possible malfunction is present due to emergence of a leakage resistance at a measurement voltage input 1 in a connection region of a measuring apparatus, or whether the measurement is providing an uncorrupted measured value.